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Influenza viruses are enveloped negative-sense RNA viruses. The segmented nature of the genome consists of 8 segments. These segments encode proteins for a variety of functions, including hemagglutinin and neuraminidase, which are two surface glycoproteins. HA allows the virus to enter the host cell, and NA helps newly formed viruses leave the infected cell.
There are two primary ways influenza viruses escape recognition from the immune system: antigenic drift and antigenic shift.
We provide influenza virus-related antigens, antibodies, and other reagents to support research and vaccine innovation.
Influenza viruses are divided into 4 types based on antigenic characteristics, with different epidemiological and pathogenic properties: A, B, C, and D.
IAV
IBV
ICV
IDV
Table 1. Different types of influenza virus. (Source: Javanian M, et al. 2021)
| Type of influenza | Symptoms | Affect | Subgroups | Epidemiology |
| A | Mild to severe | Animal and human | Divided based on the antigenic properties (16 hemagglutinin and 9 neuraminidase) | Widespread |
| B | Mild | Only human | Not classified by subtype | Not cause pandemics |
| C | Mild | Human and some animals | Not classified by subtype | Not cause epidemics |
| D | Mild | Animals, unknown in human | Not classified by subtype | Not Widespread |
Figure 1. The structure of influenza A virus. (Source: Jung HE, et al. 2020)IAV's genome consists of eight segments of negative-sense RNA encoding 11 proteins. HA, NA, and M2 are transmembrane proteins embedded in the lipid envelope. Each RNA segment forms a vRNP composed of viral RNA and NP, which associates with the RNA polymerase complex. The M1 protein is located inside the viral envelope and interacts with vRNPs. HA is responsible for receptor binding and membrane fusion. NA is involved in viral release, and the M2 ion channel helps regulate endosomal pH to allow for vRNP release. IAVs cause seasonal outbreaks and pandemics, due to the high frequency of genetic mutation and reassortment between subtypes.
Both IAV and IBV are segmented negative-sense RNA viruses with eight genome segments, but IBV has higher genomic stability than IAV. The antigenic diversity of IBV's HA and NA is lower than that of IAV, with IBV divided into only two lineages: Victoria and Yamagata. IBV lacks accessory proteins like PB1-F2. The proteins of IAV and IBV differ significantly in length, amino acid composition, and function. Currently, IBV is classified into two genetically and antigenically distinct lineages, B/Victoria/2/1987-like and B/Yamagata/16/1988-like. IBV exhibits a slower rate of antigenic drift. These differences make IAV more likely to cause pandemics, while IBV mainly causes seasonal epidemics.
Figure 2. Schematic of IBV. (Source: Koutsakos M, et al. 2016)Table 2. Differences between influenza A and influenza B viruses.
(Source: Ashraf MA, et al. 2024)
| Characteristics | IAV | IBV |
| Phylogenetics | Subtyped by NA and HA (e.g., H3N2 and H1N1) | According to HA, have two lineages: Yamagata and Victoria |
| Host range | Likely animal hosts and many animal reservoirs | Animal reservoirs not established |
| Age susceptibility | Most severe in elderly and children | Most severe in adolescence and children |
| Epidemiology | Pandemic potential, global impact | Typically causes localized outbreaks, less global impact |
| Typically causes localized outbreaks, less global impact | Range from mild to severe respiratory symptom Can lead to complications such as pneumonia and bronchitis Avian strains caused severe diseases and high death rates | Similar symptoms to influenza A but milder Less likely to cause severe complications Caused severe health problems in children |
| Immunology | M1 and NP are the major CD8+ T-cell antigens Generally, strong IFN-αβ and cytokine response HA is the major antibody target | Major CD8+ T-cell antigens are unknown strong IFN-αβ and cytokine response, details unclear HA is the major antibody target |
| Antivirals | Adamantanes used are effective NAIs effective, but resistance detected | Adamantanes are ineffective NAIs effective, but resistance detected, in some cases without apparent fitness cost |
References
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